The term “lightweight concrete” means concrete that has an in-place, air-dry density that is less than that of normal concrete. Typically, lightweight concrete is made using a lightweight coarse aggregate, such as expanded clay, slate, or shale, or blast furnace slag having a density less than that of normal stone coarse aggregate. Typically, lightweight concrete is used to increase the strength-to-weight ratio of concrete and reduce the dead load of a concrete structure, freeing up design constraints and allowing an engineer or structural designer to reduce the size of columns and other load bearing elements.
Polymers can also be used as aggregates in lightweight concrete. However, many of the most abundant and least expensive polymers are hydrophobic. The use of a hydrophobic aggregate severely limits the ability of the cement paste to form a strong interfacial bond with the aggregate, thereby reducing the strength and durability of the concrete. In addition, polymers have lower compressive strength than natural coarse aggregates, which further reduces compressive strength of concrete made using polymer aggregates.
Further, the use of lightweight aggregates can be limited by the difference in density between the lightweight aggregate and the density of a cementitious paste or mortar added to the lightweight aggregate. When density differences are high, the lightweight aggregate tends to “float” or separate from the higher density cement mortar or paste.
In addition, polymer aggregates, due to their inherent low density, light weight, and porosity, can be difficult to handle, contain, measure, and dispense through standard concrete operating equipment. In particular, the use of pellets of expanded polystyrene (“EPS”) has several disadvantages. EPS balls are typically small (about 1/16 of an inch) and are very light. They can cling to surfaces by static electricity and are difficult to keep together in a pile because of their very low weight. The slightest spill becomes an ecological nightmare, as the raw material literally moves with the wind and because of its size, is very difficult to contain. The material does not readily decompose and becomes a permanent part of the ecology. The handling and placement of about 5 (solid volume) cubic feet of this material into each cubic yard of concrete poses a formidable handling problem, as this volume only weighs about 10 to 15 pounds and is difficult to measure and dispense through standard bins and scales. Complicating all of the above, a delivery truck drum typically vents its air volume as it is being loaded, blowing the polystyrene out into the environment.
Further, some applications of concrete benefit from a composition which dries relatively quickly. For example, where a poured concrete slab is used as a floor of a building, it is often necessary to allow the concrete to dry before placing a coating on the concrete surface. Such evaporation can take several weeks, months, or even a year or more in extreme cases to be complete. Any attempt to place flooring material over the moisture laden concrete slab can result in problems. For example, the adhesive material used to hold the flooring (e.g., tile, hardwood, carpeting, etc.) in place may not stick or adhere well to the moisture laden concrete. In addition, as the moisture within the slab is released into the surface, it can cause alkali accumulation, mold, mildew, delamination, or other deterioration problems with flooring or other materials placed affixed to the moisture laden concrete.
Lightweight concrete is particularly challenging when fast surface drying is desired. That is because the highly porous internal structure of typical lightweight aggregates can, absorb up to 25% or more by weight of water. This water is additive to the normal water included for ease of handling and hydration of the cementitious binder and can impart an additional amount to the concrete mix equal to 3 to 4 times that which must normally evaporate, thereby increasing the time-to-dry for adhesive or epoxy application. This additional time is beyond the tolerance of many fast-track construction schedules and increases the likelihood of bond failure should the drying time period be truncated.
Further, pressures encountered in concrete placement often approach 1000 psi gage pressure within concrete pump lines, forcing excess water into unfilled pores of typical lightweight aggregates. After the return to normal pressure, some of this water escapes the lightweight aggregate and enters the surrounding concrete mix, which can undesirably increase fluidity and deleteriously raise the water-to-cement ratio in the short term and/or remain in the hardened matrix, further contributing to longer drying times.
Accordingly, there has been a long felt need for improvements in lightweight concrete compositions and methods relating to the manufacture and use of lightweight concrete aggregates in lightweight concrete. Such lightweight aggregates should be light weight, should be easy to handle and control, and should provide suitable bond strength with cement paste found in concrete. Additionally, such compositions should not exacerbate and lengthen concrete drying times.